Even as Syria’s rebels capture more territory in their war against President Bashar Assad’s forces—including a key helicopter base on Friday—international diplomats are struggling to find a way out of the conflict, stymied by hardening positions, as each side digs in, seemingly unwilling to yield political points. The latest sign of how difficult it will be to end Syria’s war came on Friday, when Russian Deputy Foreign Minister Mikhail Bogdanov and U.S. Assistant Secretary of State William Burns met in Geneva with U.N. envoy Lakhdar Brahimi, in yet another attempt to craft a political plan on which all could agree. “Optimism and pessimism are difficult to define,” Brahimi told reporters as he tried to dodge a question about the mood of the daylong meeting in Geneva. “We are trying to find an opening to begin to aid the Syrians in this terrible situation.”

As the talks showed, that will not be easy. Just last month, Bogdanov informally broke with Moscow’s long-held support for Assad, bluntly stating what Western leaders had long regarded as obvious: That the rebels appeared headed for inevitable victory, even if it takes a while; shortly after, President Vladimir Putin said Russia was “not that preoccupied” with Syria’s regime, hinting that he believed Assad was a spent force. But while those remarks suggest that Russia was edging away from its critical military and financial support for its closest Arab ally, Russian officials insist that they will not push for Assad’s removal.

In contrast to Russia’s support, President Obama last month formally recognized the exiled Syrian National Coalition as the true representatives of the Syrian people. U.S. and European diplomats have also stated that Assad has no place in Syria’s future. After months of shuttling between Moscow and Damascus, Brahimi finally voiced the same opinion on Thursday, telling reporters that Assad would “surely not be a member” of any transitional government, set up under an international agreement drawn up last year. “I think that what [the Syrian] people are saying is that a family [the Assads] ruling for 40 years is a little bit too long,” Brahimi told the BBC. “The change has to be real.”

After countless meetings and conferences in the Middle East and Europe, there is no clear international game plan, however. And on Thursday Assad effectively ended his cordial relationship with Brahimi, accusing the former Algerian Foreign Minister of “flagrant bias” towards the rebels. That has put in doubt Brahimi’s ability to try to broker a peace deal. “If Brahimi does not get support in the next few days, his mission I think is over,” Salman Shaikh, director of the Brookings Doha Center, and a Syria expert, told TIME by phone from London, where Syrian opposition leaders met Western diplomats this week, to try forge an emergency humanitarian plan. “This is the moment of truth,” he said.

While international diplomacy inches along at a snail’s pace, the rebels are gaining far faster on the battlefield. On Friday, commanders claimed to have captured the key helicopter base of Taftanaz in the northern province of Idlib, which they had described as crucial for their ability to establish an area safe from bombing attacks by Assad’s forces.

Such an area is increasingly necessary, as millions of Syrians struggle to endure the bitter winter, with scarce international assistance. On Friday U.N. aid agencies warned that millions of Syrians are now living in dire conditions, with worsening shortages of food and fuel and precious little electricity. “Violence has left four million people inside Syria in desperate need of help,” the heads of the World Food Program, the U.N. refugee agency and UNICEF, wrote in a joint opinion piece posted on CNN.com. The conflict, they said, “has uprooted two million inside the country and sent 600,000 fleeing the horrors of war into neighboring countries.”

Added to the misery is the fact that only a handful of foreign aid workers have dared venture into rebel areas, which are under daily bombardment. On Thursday, Doctors Without Borders, one of few such, said in a statement that injured Syrians now have few options for treatment in rebel-held areas. Describing a visit to a city in Idlib province, which he did not name, the organization’s emergency operations manager Mego Terzian said, “People are stepping up to act as nurses or even surgeons, because there is simply nobody else to do it. Faced with the seriousness of the injuries and the risks involved in evacuating patients, many of the wounded are dying because they are not getting treatment or cannot be evacuated in time.”

The crisis could perhaps be eased if aid can be funneled through Syria’s exiled opposition leaders, who were anointed as the sole political representatives by dozens of Arab, European and African countries at a conference last month in Morocco. The coalition has formed a humanitarian group, based in Cairo, to coordinate millions of dollars in international aid for Syrians, which opposition leaders say has been slow in coming. “We’re coordinating with people inside Syria to solve problems,” George Sabra, a coalition leader, told TIME by phone from Istanbul on Friday. “Unfortunately the international community seems not to care about our difficulties.”

In fact, the opposition leaders credibility is on the line, too. Despite their international recognition, the coalition has so far remained outside Syria, despite Western leaders urging them to enter rebel-held areas and begin working on the ground. Many diplomats have said they believe that is essential for the exiled opposition to build relationships with the local commanders, and to bolster their credentials to rule Syria once Assad goes—a political solution which Western leaders are strongly hoping for. “The coalition is trying to build a mechanism with the local councils,” says Shaikh, of the Brookings Doha Center. “But if they’re not on the ground in a month or two and cannot show tangible results, people will forget the coalition,” he says. “And meanwhile, people are freezing to death, literally.”